This application claims the priority of PCT International Application No. PCT/EP99/06591, filed Sep. 7, 1999 (07.09.99) and German patent document 198 46 302.2, filed Oct. 8, 1998, the disclosure of which is expressly incorporated by reference herein.
The invention is directed to a method and apparatus for detecting and measuring small periodic wave patterns in technical surfaces, of the type disclosed, for example, by U.S. Pat. No. 3,850,526.
On finely processed workpiece surfaces, it is possible to observe unintended wave patterns of the type of interest here, with a period up to about 1.5 mm; and the possibility exists that there may be even longer periods for wave patterns of the type in question. Accordingly, the term “small wave patterns” will be used here to denote those having a spacing between the wave-structure peaks smaller than two millimeters, although in most cases the periods are significantly less than one millimeter.
For reliable sealing at points where shafts pass through housing walls, in addition to the sealing ring provided with an annular radial sealing lip, it is also necessary to take into account the properties of the opposed running surface on the shaft. To produce a smooth surface at such shaft journals, they may be circumferentially ground, finished on a lathe, burnished, rolled or externally abraded. Besides specific roughness values, the manufacturer of the shaft journal also prescribes the degree to which the surface is free from torsion. Torsion-free, for example in the case of ground surfaces, means that the ground structure lies precisely in the circumferential direction and there are no superimposed regular wave components. However, in modern mass production it is not only difficult to create technical surfaces reproducibly without torsion, but problems are also encountered in reliably detecting such freedom from torsion and, where necessary, quantifying any torsional structure.
German patent document DE 197 40 141 A1 describes a mechanically operating test-score method for detecting and quantifying torsional structures or wave patterns that are of interest here, permitting reliable and quantitatively comprehensive information concerning virtually all relevant parameters of the torsional structure. The known test-score method gives usable results, in particular even if the wave pattern is not very pronounced and/or has a strong stochastic roughness component superimposed on it, when the stochastic roughness component has been substantially removed by means of an autocorrelation of the surface data. However, the known method of generating the surface data is very time-consuming and must be carried out very carefully. This method cannot therefore be used to monitor directly the manufacturing of technical surfaces. Instead, this method can be used as a reference method to control other torsional determination methods.
German patent document DE 198 09 790 A1, which was published after the priority date of the present patent, describes a contactless, optical method in which the raw data of the surface to be analyzed are obtained from direct, magnified imaging of a small segment of the surface. The method operates with arbitrary light as the primary-light type. The illuminated surface is sharply focused on a matt panel or on a high-resolution photodiode array by the use of imaging optics. In this method, coarse and strongly inclined torsional structures can be detected directly from the visual appearance of the image, without further processing of the image data. Such strongly pronounced torsional structures, however, can be detected by a trained eye anyway, i.e. merely with suitable illumination and with the aid of a lens. Such coarse cases generally occur rarely in practice, and do not cause problems with regard to their detection. For less pronounced and/or less inclined torsional structures, however, according to the this method it is necessary to generate image-data records of a plurality of neighboring surface parts, and to combine them as a function of position to form a uniform image-data record, each individual image-data record being respectively subjected beforehand to a Radon transformation.
Although the torsional determination method in German patent document to DE 198 09 790 A1 operates faster than the test-score method, it nevertheless also requires some degree of care since, on the one hand, the reference position of the workpiece with respect to the measuring instrument must be known accurately and, on the other hand, the measuring device and the workpiece must be held exactly stationary relative to each other during the measurement. In the known relative position of the workpiece axis and the measuring instrument, it is necessary to take a plurality of recordings in close succession, which is also time-consuming and requires care. Furthermore, the evaluation also requires some degree of care and supervision since the individual images must be matched to one another with a view to uniform average grey-scale distribution. The known method does provide information about the spacing of the wave patterns and their inclination with respect to a reference direction. However, information about the depth of the wave patterns and their cross section cannot be obtained directly using this method and can only be obtained with restrictions. The known method can be used in the laboratory to detect the presence or absence of surface torsion in a limited number of prepared workpieces, but the method is less suitable for use in manufacture.
U.S. Pat. No. 3,850,526, mentioned previously discloses a method and apparatus for the optical detection of periodic wave patterns in finely processed workpiece surfaces, in which the latter are illuminated with a beam of monochromatic, coherent light—primary light—and a diffraction image of the periodic wave patterns is produced in the secondary light returned by the surface. In this case, the primary light is directed onto the workpiece surface at a large angle of incidence with respect to the normal to the surface and approximately at right angles to the expected periodic wave patterns, and the intensity distribution of the diffraction image is evaluated. Of the beams involved (that is, the incident primary-light beam and the emerging secondary beam), one is kept at a fixed angle of at most 80° with respect to the normal to the surface, whereas the beam or evaluation channel of the other respective beam can be moved through a relatively large angle range around the position of the reflected beam. By means of a tilting beam arrangement, the strongest diffraction orders are fully detected and measured in terms of intensity profile. By comparing the intensity distribution of the diffracting light measured using a workpiece with intensity distributions obtained beforehand using various roughness standards, the surface roughness of the workpiece to be analyzed can be found. The known type of surface inspection is not, however, applicable to ground surfaces in which two different fine shape structures only one of which is of interest, are superimposed on each other. The diffraction image of the periodic wave patterns would have specular noise of the stochastic ground structures superimposed on it to the extent that it could not be identified and therefore could not provide useful information.
U.S. Pat. No. 5,189,490 discloses a normal roughness measurement using the scattered-light method, in which a primary-light beam is reflected by the surface to be analyzed, and the reflected secondary beam is scattered in a way characteristic of the roughness structure of the surface. Using this scattered-light method, which is widespread in principle and in different configurations and/or applications, neither diffraction by a periodic diffraction structure nor alignment of the primary light at right angles to the orientation of parallel processing tracks take place. The method disclosed by the cited document indirectly provides qualitative and quantitative useful information about the surface to be analyzed only after comparison of the actual pattern of the scattered-light cone, obtained from the specimen, with a large number of stored reference patterns produced beforehand using known patterns.
U.S. Pat. No. 3,782,827 discloses a very similar comparison method, which operates according to the scattered-light method, for determining the roughness structure of technical surfaces. This measurement method only supplies a certain actual pattern of a scattered-light distribution of the respective specimen. By similarity comparison with a large number of stored reference patterns, it is possible indirectly only to find useful information as to whether or not the analyzed specimen has a similar surface topography to a known sample.
It is an object of the invention to provide an improved, manufacturing compatible, method and apparatus of the generic type, which can perform a measurement quickly, conveniently and with reproducible information being obtained.
Another object of the invention is to provide such a method and apparatus in which exact relative alignment of the workpiece and the device is no longer necessary for the measurement result.
Still another object of the invention is to ensure sufficient stability of the measurement image, even in the event of relatively unstable handling both of the workpiece and of the device.
Finally, yet another object of the invention is to provide a single measurement which gives not only reliable quantitative information, as to whether a periodic surface wave pattern is “present” or “not present”, but also qualitative information about the period and the depth of the wave pattern, where appropriate, with corresponding image-data evaluation.
These and other objects and advantages are achieved by the detection method and apparatus according to the invention, by providing monochromatic, coherent light which strikes the wave crests on the surface at right angles to their length at a large angle of incidence—approximately grazing incidence. By exploiting the diffraction by the waved surface structure as a diffraction grating, a diffraction image is produced and the intensity distribution therein is evaluated. Owing to the large angle of incidence of the light, the effect of the scattered light from the stochastic ground structure in the diffraction image is substantially eliminated. The diffraction image remains stable and stationary even if the degree to which the measuring instrument is held stationary relative to the workpiece varies. Furthermore, the quality of the diffraction image does not depend, within realistic limits, on the relative alignment of the workpiece and the measuring instrument.
The presence of a torsional structure can be detected immediately from the occurrence of a local intensity maximum in the scattered-light cone. The spacings of the wave crests can be deduced from the spacing of a plurality of maxima—where different diffraction orders are involved. In this case, the spacings of the intensity maxima vary approximately in inverse proportion to the wave-crest spacings. That is, a very close succession of wave crests causes widely spaced brightness maxima in the intensity distribution of the scattered-light cone, while the intensity maxima are closer together if the spacings of the wave crests are larger. It is even possible to deduce the depth of the wave troughs by evaluating the intensities of the different diffraction orders and the period. However, it is difficult to obtain information about the angular position of the wave patterns in relation to the exact circumferential direction in the present invention.
Before explaining the invention further, a few comments will be made regarding the terminology. On the one hand, there are repeated references to “primary light” and “secondary light” and, on the other hand, there are references to a “diffraction image” in the secondary beam. The term “primary light” used here means light which is sent onto the workpiece surface to be analyzed (that is, incident light). The “secondary light” is not, for instance, light due to a fluorescence phenomenon, but the light which is returned, scattered, diffracted or reflected by the workpiece surface. Furthermore, despite the fact that the term “diffraction image” is used, it should not be inferred that an image forming method or device is involved in the present case. The primary light is diffracted by the waved surface structure and an intensity distribution corresponding to the diffraction is created in the secondary beam. “Diffraction image” means the specific nature of the relevant intensity distribution of this diffraction. A pictorial representation of the “diffraction image” for evaluating this intensity distribution (e.g., by using lenses or the like which form images) is not only unnecessary but would be incorrect. The direct collection of the “diffraction image” on a matt panel is sufficient for the visual intensity evaluation.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.